Investments Available-for-Sale Securities in a Loss Position Narrative (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) Rate	Dec. 31, 2017 USD ($) Rate
Schedule of Available-for-sale Securities [Line Items]
Total estimated fair value	$ 281,831	$ 214,211
Total gross unrealized holding losses	6,122	2,475
Gross unrealized holding losses, less than 12 months	(2,033)	(558)
Other-than- temporary impairment losses	$ 0	$ 0
Fixed maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of available-for-sale securities in unrealized loss positions	306	195
Total estimated fair value	$ 281,800	$ 203,432
Total gross unrealized holding losses	$ 6,100	$ 2,244
Number of available-for-sale securities in unrealized loss positions, greater than one year	124	126
Available-for-sale securities, percentage of securities with ratings of A minus or better | Rate	89.50%	91.10%
Gross unrealized holding losses, less than 12 months		$ (327)
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of available-for-sale securities in unrealized loss positions		13
Total estimated fair value		$ 10,779
Total gross unrealized holding losses		$ 231
Number of available-for-sale securities in unrealized loss positions, greater than one year		0
Gross unrealized holding losses, less than 12 months		$ (231)